Carol Shaw's endless river, the same every time you fly it. The cartridge rebuilt from the original ROM, one instruction at a time: every bank, bridge, fuel depot and missile behaves as the real machine plays them.
Fly up the river and shoot what gets in your way. Fuel drains the whole time: fly over a depot to fill the tank before the gauge reaches E. Touch a bank, a bridge or anything on the water and the jet goes down.
The Atari joystick has one button and eight directions, so every face button fires. Sound starts after your first click or key press; that is the browser's rule, not the cartridge's.
The gauge under the river runs from E to F. It drains every frame you fly, faster than it looks, and a warning tone starts once it drops below a quarter.
